It now appears that Congress will direct NASA to return Americans to the surface of the Moon in order to prepare for future crewed journeys to the surface of Mars. But the Moon could prove to be much more than just a beyond LEO testing ground for astronauts and components.

At minimum, an interplanetary round trip to the Red Planet would
require several hundred tons of water for drinking, washing, food
preparation, radiation shielding, the production of air, and for the
production of liquid oxygen (LOX) and liquid (LH2) for rocket
propellant.

Supplying the water needed for an interplanetary spacecraft parked at LEO would require a delta-v ranging from 9.3 km/s to 10 km/s from the Earth's surface. Minimum energy launch windows during the 2030s would require an additional delta-v ranging from 3.59 km/s to 4.81 km/s for Trans Mars Injections (TMI) that could send humans to Mars in less than a year.

But at different locations within cis-lunar space, substantially lower levels of delta-v could be taken advantage of in order to launch cargo and crew into a Trans Mars Injection. And these interplanetary launch points within cis-lunar space are most easily accessible from the lunar surface rather than from within the Earth's deep gravity well. So utilizing lunar ice resources could greatly reduce the cost and the complexity of sending humans to Mars.

Delta-v to important destinations within cis-lunar space

Earth surface to LEO - 9.3 km/s to 10 km/s

LEO to EML1 - 3.77 km/s (~3 days)

LEO to EML1 - 4.5 km/s (~2 days)

LEO to EML2 - 3.43 km/s (~8 days)

LEO to EML2 - 3.95 km/s (~4 days)

Lunar surface to EML1 - 2.52 km/s (~3 days)

Lunar surface to EML2 - 2.53 km/s (~3 days)

Delta-v from cis-lunar space to Trans Mars Injection

LEO to TMI (2030s) - 3.59 km/s to 4.81 km/s

EML1 to TMI (2030s) - 1.04 km/s to 1.3 km/s

Because of its shorter travel times from Earth orbit, EML1 would appear to be the optimal region for locating Deep Space
Habitats (DSH), reusable lunar shuttles, propellant manufacturing water
depots that receive water from the lunar surface and for launching
cargoes and crew on interplanetary journeys to the orbits of Mars and
Venus.

A delta-v of 3.77 can transport crews to EML1
in approximately 4 days. It would require 3.95 km/s to travel to EML2 in
4 days but 8 days of travel and radiation exposure would be required to
take advantage of a lower delta-v to EML2 at 3.43 km/s. A delta-v of
2.52 km/s would be required for a 3 day journey between EML1 to the
lunar surface. 2.53 km/s would be required for a similar journey between
EML-2 and the lunar surface. The significant presence of habitats and
depots and reusable vehicles at EML-2 could also interfere with future
radio astronomy on the back side of the Moon.

Taking full advantage of polar ice resources on the Moon would require a space architecture that utilizes orbiting depots to supply water and propellant to reusable interplanetary spacecraft and landing vehicles. Some space advocates have argued that propellant depots make heavy lift vehicle's an unnecessary expense. However, heavy lift vehicles would make it possible to easily deploy propellant manufacturing water depots to EML1 for crewed interplanetary journeys to the orbits of Mars and Venus.

Similar depots located at LEO could also enable large payloads originally deployed to LEO by heavy lift vehicles to be later deployed by reusable orbital transfer vehicles practically anywhere within cis-lunar space.

Expendable EUS for SLS launch vehicle (Credit: NASA)

In 2018, NASA will launch the first SLS heavy lift vehicle with an unmanned Orion space capsule and a hypergolic fueled Service Module and an interim upper stage. But NASA currently envisions crewed SLS launches in the 2020s to include an Orion space capsule with a hypergolic fueled Service Module and a large LOX/LH2 fueled Exploratory Upper Stage (EUS).

But two of the principal companies (Boeing and Lockheed Martin) currently developing the SLS/Orion architecture are also-- privately developing-- an alternate space architecture through their joint company, the ULA (the United Launch Alliance). In this alternate architecture, the ULA will utilize their emerging IVF (Integrated Vehicle Fluids ) technology to deploy a reusable ACES upper stage that could eventually utilize LOX/LH2 propellant depots for travel within cis-lunar space during the 2020s. IVF technology allows a spacecraft to utilize hydrogen and oxygen ullage gases for attitude control, power production, and for autogenously pressurizing propellant tanks, eliminating the need for hydrazine and liquid helium.

NASA could greatly enhance the capability, efficiency, and the safety of the SLS/Orion architecture by taking full advantage of the ULA’s emerging IVF technology. While there's no reason to stop Boeing from developing the EUS as an expendable upper stage for the SLS, it would still be technologically advantageous for NASA to commission the ULA to use its IVF technology to convert some EUS vehicles into reusable orbital transfer vehicles and others into propellant depots.

An SLS/Orion architecture utilizing EUS derived Orbital Transfer Vehicles (OTV-125) and EUS derived propellant producing water depots (WPD-OTV-125) would no longer require the expense and the enhanced risk of launching astronauts on top of a super heavy lift vehicle. So for beyond LEO missions in the 2020s, under this scenario, astronauts would simply be transported to LEO by Commercial Crew vehicles that would have already been in operational service since 2018.

Once in orbit, the commercial crew capsule would dock directly with the Orion-OTV-125 reusable spacecraft, transferring its crew aboard the Orion for its beyond LEO mission. Alternatively, a crew capsule could dock at the port of a space station where
the Orion-OTV-125 would also be docked at a different port. Both scenarios would mean that the ATV based hypergolic Service Module being manufactured by the Europeans would not be required for crewed beyond LEO missions, and would only be used once during the 2018 test mission. So only the domestically manufactured Orion capsule being developed by Lockheed Martin would be preserved in this architecture.

A single SLS Block I cargo launch could be used to deploy two WPD-OTV-125 depots to LEO plus a two 1 MWe solar arrays. With at least 35 tonnes of propellant, one of the water/propellant depots could self deploy itself to EML1 along with its 1.4 to 2.8 MWe solar array. Another basic SLS Block I cargo launch could be used to deploy two reusable Orion-OTV-125 vehicles to LEO. But once this reusable depot based extraterrestrial architecture is deployed by the SLS, private commercial launch vehicles will only be required to conduct crewed beyond LEO missions. This will allow the SLS to be used exclusively as a cargo launcher for the deployment of large and heavy spacecraft and habitats and other large and heavy structures.

Water for the propellant manufacturing orbital depots could be regularly supplied to LEO and to EML1 from commercial launch vehicles (Atlas V, Delta IV heavy, Falcon 9, and the future Falcon Heavy, Vulcan, and Vulcan Heavy vehicles). However, once water is being manufactured on the lunar surface, water transported to EML1 would be supplied exclusively from the lunar surface.

Since the Orion-OTV-125 would derived from the EUS, it would be capable of accommodating up to 125 tonnes of propellant. But less than 35 tonnes of propellant would be required for an Orion-OTV-125 to transport its crew to EML1. And an equal amount of fuel would be needed to return astronauts to LEO. No aerobraking would be required.

A reusable Extraterrestrial Landing Vehicle (ETLV) would transport astronauts from EML1 to the surface of the Moon and back to EML1 with a single fueling of LOX/LH2. A single SLS Block I cargo launch could deploy three ten tonne ETVL-2 vehicles to LEO, each with enough propellant to travel to EML1. The ETLV-2 could also serve as an back up OTV in case one or both of the Orion-OTV-125 vehicles becomes inoperable.

Until new RS-25 engines are in production, perhaps by 2021 to 2023, NASA will only be able to launch four SLS vehicles. And one of those launches will be an unmanned test launch in 2018. So after 2018, only three SLS launches will be possible before the RS-25 engines go into production.

But just three basic SLS Block I cargo launches would be required to deploy a reusable EUS derived cis-lunar transport architecture (WPD-OTV-125, Orion-OTV-125, and the ETLV-2) capable of transporting humans to EML1 and to the lunar surface and back.

However, the deployment of the lunar landing vehicles could be delayed until the new RS-25 engines are in production. This would allow NASA to use their last four Space Shuttle Main Engines to be used to launch a DHS (Deep Space Hab) to EML1. An SLS propellant tank derived habitat with over 510 cubic meters of pressurized habitable volume would have a dry weight of 22.4 tonnes. So in theory, a single SLS Block I cargo launch could be used to deploy two or three pressurized habitats to LEO. A partially fueled Orion-OTV-125 could then be used to transport one of the pressurized habitats to EML1.

SLS propellant tank derived habitat (Credit: NASA).

This would leave one or two pressurized habitats at LEO with a combined pressurized habitable volume exceeding that of the ISS! All of that would be achieved with a single SLS launch and a reusable propellant producing water depot architecture routinely and sustainably supplied with water from commercial launch vehicles.

"The knowledge that we have now is but a fraction of the knowledge we must get, whether for peaceful use or for national defense. We must depend on intensive research to acquire the further knowledge we need ... These are truths that every scientist knows. They are truths that the American people need to understand." (Harry S. Truman 1948).